Multiphase flow of liquid such as oil and/or water, and gas, is almost always involved in the production of hydrocarbons from subsurface formations. Upward multiphase fluid flow in a well can often lead to flow stability problems.
Production instabilities can be encountered for example in the form of large fluctuations of the oil production rate, e.g. more than 25% of the average production rate, or in situations where big slugs of oil are alternated by gas surges. Particular problems are encountered in gas-lifted wells, in which gas is injected from surface via a casing/tubing annulus and an injection valve upstream in the well into the production tubing. Also here, severe instabilities of the gas/liquid ratio of the fluid produced up the tubing can occur. A special problem is observed in dual gas-lifted wells, wherein two tubings are arranged, usually with inlets for reservoir fluid at different depths. A common problem there is that production through one of the tubings ceases, due to instabilities in the lift gas injection into the tubings.
Such instability phenomena are often referred to as “heading”, e.g. tubing heading, casing heading etc. Heading is generally undesirable, not only because of production loss, but also because downstream fluid handling equipment such as separators and compressors can be upset, damaging of the wellbore or flowline, and negative influences on other wells connected the same equipment.
Several systems and methods have been proposed in the past to control heading phenomena.
International patent application with publication No. Wo 97/04212 discloses a system for controlling production from a gas-lifted oil well, comprising a choke for adjusting the flow of crude oil from a production tubing of the well, into which lift gas is injected at a downhole position in the well. A control system is provided to dynamically control the opening of the choke, such that the casing head pressure in the lift gas injection line is minimized and stabilized.
SPE paper No. 49463 “Real-Time Artificial Lift Optimization” by W. J. G. J. der Kinderen, C. L. Dunham and H. N. J. Poulisse discloses a combination of this system with a production estimator based on a measurement of pressure drop over a fixed restriction. The pressure drop is used to estimate production, and the choke is slowly stepwise adjusted to find an optimum opening for maximum production.
USA patent publication No. U.S. Pat. No. 6,293,341 discloses a method for controlling a liquid and gaseous hydrocarbons production well activated by injecting gas, wherein a produced-hydrocarbon flow rate is estimated from the temperature measurement of the produced hydrocarbons, and is compared with four predetermined flow rate thresholds. Depending on the outcome of the comparison, and depending on the gas-injection rate and the aperture of the outlet choke, the gas injection flow rate or the aperture of the outlet choke are stepwise adjusted by a predetermined amount.
It is an object of the present invention to provide a method for controlling the flow of a multiphase fluid from a well that provides efficient and robust control in a variety of situations and with minimum requirements for control hardware.